Dielectric cooking apparatus



Oct. 5, 1965 T. LENART DIELECTRIC COOKING APPARATUS Filed March 22, 1963 2 Sheets-Sheet 1 ,3 I III! I i 0' x" l INVENTOR.

Oct. 5, 1965 "r. LENART 3,210,513

DIELECTRIC COOKING APPARATUS Filed March 22, 1963 2 Sheets-Sheet 2 INVENTOR.

United States Patent 3,210,513 DIELECTRIC CGOKING APPARATUS Tibor Lenart, Snndbyberg, Sweden, assignor to Aktiebolaget Electrolux, Stockholm, Sweden, a corporation of Sweden Filed Mar. 22, 1963, Ser. No. 267,247

Claims priority, application Sweden, Mar. 27, 1962,

3,364/62 1 Claim. (Cl. 21910.55)

My invention relates to dielectric cooking apparatus, and more particularly to dielectric cooking apparatus in which heating is effected in a cavity of an oven by microwave electrical energy developed by an oscillator, such as a magnetron, for example.

In dielectric cooking apparatus of this type, a magnetron of the continuous wave type often is employed for developing ultra high frequency electrical energy as high as 2400 to 2500 mc./s. The electrical energy, which is in the form of high frequency electromagnetic waves, referred to as microwaves, is transmitted from the magnetron through a wave guide to the cavity of an oven adapted to hold the food to be heated. A part of the microwave electrical energy received in the cavity is reflected and transmitted back to the magnetron. This is objectionable because the magnetron can be adversely affected when the reflected microwave electrical energy becomes sufiiciently great. For example, the reflected electrical energy transmitted back to the magnetron effects objectionable heating of the anode. Further, the magnetron can be electronically damaged by the reflected electrical energy for the reason that the magnetron and load in the cavity form a system equivalent to two resonant circuits which are coupled together. In the event the resonant frequency of the load is very close to the resonant frequency of the magnetron and the resistive component is small, the magnetron can be forced to oscillate on the new frequency, which will overload the cathode and quickly damage the magnetron.

The object of my invention is to construct dielectric cooking apparatus of this type which will produce the desired energy transfer from the magnetron to the cavity and substantially prevent microwave energy reflected in the cavity from being transmitted back to the magnetron. I accomplish this by providing a wave guide which is disposed between the magnetron and the cavity and constructed so that microwave energy can pass therethrough only in one direction from the magnetron to the cavity.

My invention will be more fully set forth in the follow ing description referring to the accompanying drawing, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In the accompinying drawings, FIG. 1 is a diagrammatic representation, in section, of a heating oven embodying my invention; FIG. 2 is an enlarged perspective view of the wave guide shown in FIG. 1; FIG. 3 is a perspective view similar to FIG. 2 illustrating a further embodiment of the invention; and FIG. 4 is a diagrammatic representation of a wave guide like the one shown in FIG. 2 illustrating a still further embodiment of the invention.

Referring to FIG. 1, I have shown my invention in connection with a heating oven having a cavity 1 pro vided with a shelf 2 desirably formed of material like glass, for example. The cavity 1 has an access opening ice provided with a door or closure member 3 through which material to be heated is inserted and removed from the cavity. A receptacle 4 on the shelf 2 may be employed to hold the material, such as food, for example, to be heated.

The material in the cavity 1 is heated by microwave electrical energy produced by an oscillator, such as a magnetron 5, for example, having an antenna 6 associated therewith from which microwave energy is transmitted. The antenna 6 projects into a wave guide 7. The microwave energy transmitted from the antenna 6 travels through the wave guide 7 into the cavity 1 through an opening in the cavity wall. The microwave electrical energy received in the cavity 1 in the form of traveling electromagnetic Waves produces an electromagnetic field, whereby heating of the food by dielectric losses is effected. Below the shelf 2 a stirrer 8 is arranged driven by a motor 9. The electromagnetic field in the cavity 1 is made as homogeneous as possible by the stirrer 8.

The cathode C of the magnetron is heated by current supplied through conductors 11, 12 from a heating transformer 10. A high voltage supply 13 delivers the necessary energy to the anode A of the magnetron 5. One conductor 14 from the supply 13 is connected to the conductor 12 and the other conductor 15 is connected to the anode A. A magnetizing coil 16 having terminals 16a, 16b is connected in series with the anode A in the conductor 15.

In accordance with my invention, I provide the wave guide 7 through which microwave energy can pass in only one direction from the magnetron 5 to the cavity 1 and in which microwave energy reflected in the cavity 1 is prevented from being transmitted back to the magnetron 5. Referring to FIG. 2, the wave guide 7 comprises a circular wave guide section 17 provided with an element 18 formed of ferrite. The element 18, which is in the form of an elongated rod having pointed ends, is mounted coaxially within the circular section 17 intermediate its ends. The electromagnet or magnetizing coil 16 is disposed about the circular section 17 to produce a magnetic field for magnetizing the ferrite element 18.

At each end of the circular section 17 are provided end sections 19 and 20 which generally are rectangular in cross-section and have wide and narrow sides, respectively. The inner ends of the end sections 19 and 20 are joined to the ends of the circular section 17, the inner ends of the wide sides of the end sections flaring outward at transition zones between the circular section and rectangular end sections joined thereto. The end section 19, which may be referred to as a rectangular input wave guide, is coupled to the magnetron 5 while the end section 20, which may be referred to as a rectangular output wave guide having an outer open end 21, is directed toward cavity 1 and situated at the opening in the cavity wall.

The rectangular input and output wave guide sections 19 and 20 are angularly offset with respect to one another about the longitudinal axis of the wave guide 7. In the embodiment shown in FIG. 2, the rectangular output wave guide section 20 is angularly turned 45 in a counterclockwise direction with respect to the rectangular input wave guide section 19 when the wave guide 7 is viewed axially from the end of the input wave guide section 19.

When the dielectric cooking apparatus in FIG. 1 is being operated to heat material, such as food, in the cavity 1, the material in the receptacle 4 will be subjected to direct radiation and eflective heating thereof will be promoted by the electromagnetic waves passing into the cavity 1 from the end opening 21 in the wave guide 7. The magnetron 5 emits microwaves which pass through the rectangular input wave guide section 19. The plane of polarization of the incident waves emitted by the magnetron 5 is normal or perpendicular to the wide side of the rectangular input wave guide section 19 and parallel to the arrow 22 in FIG. 2. The antenna 6 of the magnetron 5 is in the shape of a rod parallel to the arrow 22.

When the magnetizing coil 16 is energized, a magnetic field is produced in the circular wave guide section 17, such magnetic field having lines of flux passing axially through-the ferrite element 18. The plane of polarization of the microwaves is rotated magnetically by the ferrite element 18 so that the plane of polarization of the waves is normal or perpendicular to the wide side of the rectangular output wave guide section 20 and parallel to the arrow 23 in FIG. 2. Accordingly, the plane of polarization of the microwaves is rotated about 45 in a counterclockwise direction when the wave guide 7 is viewed axially from the end 19a of the input wave guide section 19. The microwaves can then pass through the output wave guide section 20 and travel from the open end 21 thereof to the cavity 1 without attenuation or loss of energy.

Microwaves reflected in the cavity 1 pass into the rectangular output wave guide section 20. The plane of polarization of the reflected microwaves received by the output wave guide section 20 is perpendicular to the wide side thereof and parallel to the arrow 23 in FIG. 2. The plane of polarization of the reflected microwaves is rotated magnetically by the ferrite element 18 so that the plane of polarization of the reflected waves is substantially at a right angle or about 90 to the arrow 22 in FIG. 2. Accordingly, the plane of polarization of the reflected microwaves is rotated about 45 in a counterclockwise direction when the wave guide 7 is viewed axially from the end 19a of the input wave guide section 19. The reflected'microwaves are attenuated and substantially extinguished when the plane of polarization of the reflected waves is magnetically rotated and at an angle of about 90 with respect to the arrow 22.

The magnetron 5. is extremely sensitive to voltage fluctuations. For example, the energy output of the magnetron is reduced about fifty percent when the voltage of the source of electrical supply 13 decreases about fifteen percent. The wave guide 7 can be eflectively employed to control heating of material in the cavity 1 in spite of fluctuations in voltage of the source 13 of electrical supply. The magnetizing coil 16 for the wave guide 7 is connected in conductor of the anode circuit in series with the anode A. In addition, the magnetizing coil 16 is connected by conductors 24 and 25 to an independent source of electrical supply 26.

The plane of polarization of the incident microwaves is rotated by the ferrite element 18 through an angle which is proportional to the value of the exciting magnetic field produced by the magnetizing coil 16. When the magnitude of the current flowing through the coil 16 increases and decreases, the angle through which the plane of polarization of the incident microwaves rotates respectively increases and decreases. When the magnitude of the anode current changes, there also is a change in the current for the coil 16 and the value of the exciting magnetic field produced by the coil. With this arrangement, the plane of polarization of the incident microwaves passing into the circular section 17 through the input wave guide section 19 may be rotated through an angle smaller or greater than 45 Under such conditions, the plane of polarization of the microwaves is rotated magnetically by the ferrite element so that the plane of polarization of the waves is not parallel to thearrow 23 but instead is inclined thereto. Under these conditions, the microwave energy passing through the output wave guide section 20 to the cavity 1 is reduced and less than that passing therethrough when the plane of polarization of the waves is parallel to the arrow 23.

By energizing the wave guide '7 in the manner shown in FIG. 1 and just described, the wave guide can be effectively employed to stabilize the operation of the heating apparatus so that if there are fluctuations in voltage, foods and food preparations can be heated in the same length of time as they would be if there were no fluctuations in voltage of the source of electrical supply 13. The magnitude of the exciting magnetic field produced by the coil 16 when energized from the independent source of electrical supply 26 can be chosen so that the rotation of the plane of polarization of the incident waves passing through the input wave guide section 19 will be dependent upon variations in the anode current resulting from voltage fluctuations of the source of electrical supply 13, whereby the microwave energy passing through the output wave guide section 20 can be nicely controlled.

The microwaves reflected in the cavity 1 and prevented from reaching the magnetron 5 in the manner explained above oscillate in the circular wave guide section 17, that is, are reflected back and forth between the openings of the rectangular input and output wave guide sections 19 and 20, respectively. Heat is generated by the oscillating microwaves and efiects heating of the circular wave guide section 17 and ferrite element 18 and mounting components therefor. In order to dampen the oscillation of the reflected microwaves in the circular wave guide section 17 and prevent the generation of heat, a wave guide may be employed like that shown in FIG. 3 in which parts similar to those illustrated in FIG. 2 are designated by the same reference numerals. In FIG. 3 the wave guide 7a includes a circular wave guide section 17a having an opening 27 over which a wave guide 28 is positioned. As explained above, the microwaves reflected in the cavity 1 travel through the output wave guide section 20 into the circular wave guide section 170 and the plane of polarization of the reflected waves is rotated magnetically by the ferrite element 18 so that the waves cannot pass through the input wave guide section 19 and be transmitted back to the magnetron 5. However, the wave guide 28 is coupled to the circular wave guide section 17a in such manner that the reflected waves, after the plane of polarization of such waves is rotated magnetically by the ferrite element 18, can pass through the opening 27 into the wave guide 28. A suitable material 29, such as a dielectric loss material, for example, is provided in the wave guide 28 for absorbing the reflected microwave energy passing through the opening 27. In this way, the material 29 functions to dampen the reflected microwave energy oscillating in the circular wave guide section 17a. Of course, there is a magnetizing coil in the FIG. 3 embodiment like the one shown in FIG. 2. In order to make the other details more clear in FIG. 3 the coil has not been shown.

The wave guide 28 essentially constitutes a short-circuited wave guide in which the impedance of the material or load 29 is matched to that of the reflected microwave energy transmitted to the circular wave guide section 17a, whereby the load is capable of absorbing and consuming all of the microwave energy reflected in the cavity '1 and passing'through the output wave guide section 20 to the circular wave guide section 17a. When the plane of polarization of the reflected microwaves is rotated magnetically by the ferrite element 18 through an angle of 45, the plane of polarization of the waves is parallel to the wide side of the rectangular input wave guide section 19 and with respect to the arrow 22. When the plane of polarization of the waves in the circular wave guide section 17a normally is rotated magnetically through an angle of 45 as in the embodiment.

of FIG. 3, the wave guide 28 desirably is positioned on the circular wave guide section 17a so that planes passing horizontally therethrough and perpendicular to the opposing parallel wide sides 30 of the wave guide 28 are essentially parallel to the wide side of the rectangular input wave guide section 19. Also, the wave guide 28 desirably is positioned on the circular wave guide section 17a with its parallel wide sides 30 parallel to the longitudinal axis or center line of the wave guide 7a. In order for the circular wave guide section 17a to show an open circuit for the reflected wave at the opening 27, the wave guide 28 must be separated from the left hand end of the wave guide section 17a a given distance which is about equal to half the wave length, that is, 2M4, or 4M4, 6M4 etc.

Another way to dampen the oscillation of the reflected.

microwaves and prevent the generation of heat in the circular wave guide section 17 is diagrammatically illustrated in FIG. 4. In FIG 4, the longitudinal axis of the wave guide is represented by the line 31, the ferrite element 18a is mounted coaxially of the wave guide at the zone of the circular wave guide section, and the rectangles 32 and 33 represent the ends of the rectangular input and output wave guide sections 19a and 20a corresponding to the wave guide sections 19 and 20, respectively, in FIG. 2.

Damping members 34 and 35 are provided along the center line 31 at the ends of the ferrite element 18a to absorb the reflected oscillating microwaves in the circular wave guide. The damping members 34 and 35 comprise sheets formed of dielectric loss material, such as metal sheets covered with a thin layer of graphite, for example. The sheets 34 and 35 are transverse to the longitudinal axis of the wave guide which is represented by the center line 31 and placed inside the circular wave guide section 17. The sheet 34 extends crosswise within the intermediate circular wave guide section with its major axis 36 parallel to the wide side of the rectangular input wave guide section 19a, and the sheet 35 is at an angle of 45 With respect to the sheet 34 with its major axis 37 parallel to the wide side of the rectangular output wave guide section 20a.

When the plane of polarization of the reflected waves in the circular Wave guide section is rotated magnetically through an angle of 45 by the ferrite element 18a, the plane of polarization of the reflected waves is parallel to the major axis 36 of the sheet 34. Under these conditions, the greatest part of the reflected microwave energy is absorbed in this plane by the sheet 34. The reflected microwaves not absorbed by the sheet 34 are reflected therefrom toward the sheet 35, because such reflected microwaves are prevented from being transmitted through the rectangular input wave guide 19a for the reasons given above in describing the operation of the embodiment of FIG. 2. The plane of polarization of the reflected microwaves in the circular wave guide section that are not absorbed by the sheet 34 and are reflected therefrom is rotated magnetically by the ferrite element 18a, the plane of polarization being rotated so that it is parallel to the major axis 37 of the sheet 35. Under these conditions, the reflected microwaves are reflected across the circular wave guide section from the sheet 34 and are effectively absorbed by the sheet 35. The oscillating microwave energy now remaining in the circular wave guide section is relatively small and negligible and can be neglected. The microwaves emitted from the magnetron 5 and passing successively through the input and output wave guide sections 19a and 200" respectively, will be absorbed to a very small extent only by the energy absorbing sheets 34 and 35, because the plane of polarization of the incident waves in the input Wave guide section 1942 is parallel to the arrow 22a and perpendicular to the major axis 36 of the 6 sheet 34, and because the plane of polarization of the waves in the output wave guide section 20a and traveling to the cavity 1 is parallel to the arrow 23a and perpendicular to the major axis 37 of the sheet 35.

In view of the foregoing, it will now be understood that the wave guide 7 in FIG. 1 serves as a connection between the magnetron 5 and cavity 1, the wave guide 7 functioning as a section of the connection providing the only path for microwave energy passing from and toward the magnetron 5. The wave guide 7 serves to promote the passing of microwave energy through the connection from the magnetron 5 to the cavity 1 and to prevent microwave energy reflected in the cavity from being transmitted back to the magnetron through the connection. The electromagnet 16 when energized functions to apply a magnetic field to the axis of the circular wave guide section 17 and to the rod-shaped ferrite element 18 therein to promote passage of microwave energy from the input wave guide section 19 into the output Wave guide section 20 and to prevent passage of reflected microwave energy from the output wave guide section 20 in the input wave guide section 19.

The high voltage supply 13, which may be referred to as a first source of electrical supply, may be subject to fluctuations in voltage. The source of electrical supply 26, which is independent of the first source of electrical supply, may be referred to as a second source of electrical supply. As seen in FIG. 1, the anode A forms a part of an electrical circuit connected to the first source of electrical supply 13, and at least a part of the electromagnet 16 is connected in the anode circuit. The electromagnet 16 also is connected to the second independent source of electrical supply 26. With this arrangement the electromagnet 16 when energized applies a magnetic field to the axis of the circular wave guide section 17 and to the rod-shaped ferrite element 18 therein by electrical energy derived from both the first and second sources of electrical supply 13 and 26 to control passage of microwave energy from the input Wave guide section 19 into the output wave guide section 20 to stabilize the operation of the oven and to prevent passage of reflected microwave energy from the output wave guide section 20 into the input wave guide section 19, as explained above.

Although I have shown and described particular embodiments of my invention, I do not wish my invention to be limited to the particular arrangements set forth, and I intend in the following claim to cover all modifications which do not depart from the spirit and scope of my invention.

I claim:

Dielectric cooking apparatus comprising an oscillator for supplying microwave energy, said oscillator having an anode and a cathode, an oven having a cooking cavity, a connection between said oscillator and said cavity, said connection including a section providing the only path for microwave energy passing from and toward said oscillator, structure in said section to promote the passing of microwave energy through said connection from said oscillator to said cavity and to prevent microwave energy reflected in said cavity from being transmitted back to said oscillator through said connection, said lastmentioned structure comprising an elongated wave guide having a longitudinal axis and including a rectangular input wave guide section which receives microwave energy supplied by said oscillator and a rectangular output wave guide section which is at an acute angle with respect to said input Wave guide section about the longitudinal axis of said Wave guide and an intermediate circular wave guide section therebetween, a rod-shaped ferrite element having a circular cross-section and being coaxially disposed in said circular wave guide section, a first source of electrical supply subject to fluctuations 7, in voltage, a second source of electrical supply which is independent of the first source of electrical supply, said anode forming a part of a circuit connected to said first source of electrical supply, an electromagnet having at least a part thereof connected in said anode circuit, means for connecting said electromagnet to the second source of electrical supply, and means including said electromagnet for applying a magnetic field to the axis of said circular wave guide section and to said rod-shaped ferrite element therein by electrical energy derived from both the first and second sources of electrical supply to control passage of microwave energy from said input wave guide section into said output wave guide section to stabilize the operation of said oven and to prevent passage of reflected microwave energy from said output wave guide section into said input wave guide section.

8 References Cited by the Examiner UNITED STATES PATENTS 8/56 Fox 333--24.1

4/57 Haagensen 219-10.55 11/57 Smith 219--10.55

3/58 Haagensen 21910.55 3/60 Hahn 219-10.55

9/60 Porter 33324.1 10/60 Woodman 219-10.55 4/61 Van Uitert 333-24.1

FOREIGN PATENTS 1/52 Great Britain.

RICHARD M. WOOD, Primary Examiner. 

